Printed circuit boards construction consists of one or more layers of an electrically conductive material, usually copper, mounted on or separated by one or more layers of dielectric material, generally a mixture of fiberglass scrim and epoxy resin. Holes are drilled through the conductive layer or layers and the drilled printed circuit board is plated. The copper is etched away in selective areas, leaving a series of electrical connections between the holes and providing the basic circuitry of the printed circuit board. The nature of the printed circuit board construction, which is dictated by end use requirements, generates a number of the problem areas that arise in high speed drilling. As an example, the fiberglass scrim in the dielectric areas of the assembly can cause drill deflection and a consequent lack of accuracy in hole position. The epoxy resin of the dielectric can be a source of epoxy smear. Some of these problems can be avoided through quality control of the basic printed circuit board materials. Others can be eliminated or greatly reduced in severity through the use of drill boards.
Today drill diameters for printed circuit boards may range from a No. 97 drill of 0.004 inch diameter to a size F drill of 0.2570 inch diameter. These drills rotate in spindles at speeds that can range from 20,000 to 120,000 RPM.
Certain kinds of problems are inherent in high speed drilling of printed circuit boards. Proper use of drill boards, both entry and backup, can eliminate or minimize these problems to acceptable limits. One such problem is surface burrs. No drilling process is completely free of burring. However, high levels of flatness and hardness of drill boards reduce burring to degrees that are acceptable for many applications. The pressure of drill entry and withdrawal at high speeds can sometimes create copper delamination on the hole wall pullaway. The flatness and hardness of drill boards, both entry and backup, can essentially eliminate these problems. Additionally, the high speed drilling at or near the limits being current commercially achieved is hard on drills. Use of properly designed drill boards can extend the drill life.
Another set of problems results in hole contamination in printed circuit boards, arising from several different sources. The first problem is phenolic smear. This condition occurs when the phenolic resin used in the preparation of a drill board is insufficiently cured. The heat generated by the drill causes a partly cured phenolic to flow. As the drill is withdrawn from the hole, the fluid phenolic clinging to the drill edges is smeared to the inside of the hole surface. This requires that any drill boards utilizing a resin have to be fully cured and that there is no partly cured resin remaining to flow, regardless of the heat generated.
Another type of hole contamination is epoxy smear, which is generated in much the same fashion as the phenolic smear described above, except for the fact that the sources of material used in the preparation of the dielectric layer of the printed circuit boards, and here again, improper curing is the problem.
In the drilling of printed circuit boards to provide apertures necessary for passing component leads therethrough and for attaching the boards in place, problems have been encountered in that burrs are produced on the periphery of the aperture where the drill exits from the copper clad surface. Such a burr is unacceptable in view of the fact that it forms a hindrance to through hole plating which can entrap contaminents, which may result in cracking of the through hole plating and subsequent electrical failure. Such burrs must be removed, either by a separate sanding operation or prevented by the use of a suitable backup board.
Backup boards are disclosed in U.S. Pat. Nos. 4,269,549 and 3,700,341. The latter mentioned patent employs a core of hard pressed wood which is covered with outer layers of metallic foil. The backup board of the first mentioned patent utilizes a core of a sinusoidal material which is stated to provide cooling air passages through the board. A backup board for drilling printed circuit boards should be so constructed that it does not permit pull back of material to contaminate the printed circuit board. Moreover, a backup board should provide a high degree of surface support for the printed circuit board resting on top thereof, and should provide a good degree of surface hardness as well as providing a core which permits the drills to terminate in a forgiving substrate.
Accordingly, the present invention provides a new and improved backup board for drilling printed circuit boards.